An example of conventional rotary cutter is disclosed in JP-Y-57-54780 and shown in FIG. 9a of the accompanying drasings. As shown, the rotary cutter B includes a body 90 with a handle 90a that extends substantially straight. The body includes a tip end attached with a discal blade 91 for cutting and a cover 92 for covering the outer circumferential cutting edge of the discal blade 91. The discal blade 91 is rotatably supported by non-illustrated a supporting shaft that. On the other hand, the cover 92 includes a rear portion formed as an operating member 92a. The operating member 92a is operated by a finger to move back and forth longitudinally of the handle 90a. 
When the rotary cutter B out of use, as shown in FIG. 9a, the cover 92 is pushed forwardly. In this state, any part of the discal blade 91 does not protrude beyond the cover 92 radially thereof. As a result, it is possible to reduce the risk that the user's fingers accidentally touch the cutting edge of the discal blade 91. On the other hand, as shown in FIG. 9b, when the operating member 92a is operated to backwardly move the cover 92, a part (amount s1) of the discal blade 91 forwardly protrudes beyond the cover 92. As a result, it is possible to cut a desired object using the discal blade 91.
According to the above prior art, the cover 92 moves backwardly while the discal blade 91 protrudes beyond the cover 92 at the front end of the rotary cutter B. Due to this structure, when the protruding amount s1 is not large enough, the handle 90a of the rotary cutter B should be held at a substantially perpendicular to a cutting target 99 for cutting, as shown in FIG. 10. As a result, difficulty arises in cutting if the handle 90a is inclined, for the convenience of cutting. Further, if the rotary cutter is used together with a ruler, the rotary cutter should be preferably inclined for facilitating movement along the ruler. However, as described above, the inclined posture of the handle 90a poses difficulty in cutting, making it inconvenient to use the rotary cutter in combination with a ruler.
On the other hand, if the protruding amount s1 of the discal blade 91 is large enough, the problem described above may be resolved. However, when the protruding amount s1 is enlarged, discal blade 91 protrudes excessively beyond the cover 92. Such a structure increases the risk that the user's fingers accidentally touch the cutting edge of the discal blade 91. Additionally, the operating member 92a needs to have a large moving stroke for increasing the protruding amount s1.